This invention relates generally to the preparation of admixtures from thermoplastics. More specifically, the invention relates to the addition to a polymer melt of additives in a partially volatile liquid vehicle followed by removal of the vehicle.
It is known to modify the characteristics of thermoplastic materials by addition of additives. This addition normally occurs prior to the shaping step, that is, prior to spinning or other extrusion, injection molding, etc. For example, colorants, stabilizers, delusterants, flame retardants, fillers, antimicrobial agents, antistatic agents, optical brightness, extenders, processing aids and other functional additives are used to impart preferred properties to the host polymer to which they are added. By way of illustration, the coloring of thermoplastic articles by incorporation of pigments introduced as color concentrates is well known. For the purposes of the present application, a concentrate contains the additive at a much higher loading than is desired in the final thermoplastic article. Typically, the coloring of fibrous materials through the use of color concentrates has presented unique challenges. The reasons for this are many fold. For instance, the amount of pigmentary particles dispersed in the concentrate must be high enough to impart satisfactory color density and yet must not be so high as to interrupt the spinning process.
When adding materials to nylon 6, moisture levels in the additive have formerly been kept as low as practical. The presence of excessive levels of water causes depolymerization and reduces the molecular weight and melt viscosity of nylon 6 to the point where bubbles will form in the extrudate or where fiber can no longer be formed. As a consequence, it has been important to keep moisture levels of concentrates low.
Pigment concentrate production may be performed by preparing a dispersion of the pigment in an aqueous medium. To achieve satisfactory dispersion, a water dispersible/soluble polymer may be used. It is normal in concentrate production to flush the pigment from an aqueous dispersion to an organic phase in a separate step prior to extrusion.
Another method of preparing pigment concentrates involves the preparation of an aqueous color dispersion which is dried to remove the water. For instance, U.S. Pat. No. 4,374,641 to Burlone describes the preparation of a color concentrate in an aqueous vehicle using a dispersant which is compatible with the thermoplastic material with which the concentrate will be blended. The aqueous color concentrate is filtered and dried to remove residual water. Both of these methods require an extra step, like flushing or drying.
In the processing of plastics, it has been described that additives may be introduced into the plastic extruder, generally in a finely divided form, such as granules or powders, with devolatilization of the volatiles present within the polymer stream. In the method of U.S. Pat. No. 4,065,532 to Wild et al., the addition of major amounts of compatible liquid additives is described as being preferably done after devolitalization of the polymer.
There remains a need for a process by which additives present in liquid vehicles, which vehicles are not desirable in the final shaped polymer article, may be added to a polymer melt stream without interfering with the polymer's suitability for processing.